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BOTTOM WATER FORMATION and POLYNYAS in Adelle LAND, ANTARCTICA

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BOTTOM WATER FORMATION and POLYNYAS in Adelle LAND, ANTARCTICA Papers tendProceedings of the Royal Society of Tasmania, Volume 133(3), 2000 51 BOTTOM WATER FORMATION AND POLYNYAS IN ADEllE LAND, ANTARCTICA by Nathaniel L. Bindoff, Stephen R. Rintoul and Robert Massom (with five text-figures) BINDOFF, NL., RINTOUL, S.R. & MASSOM, R., 2000 (31 :v): Bottom water formation and polynyas in Adelie Land, Antarctica. In Banks, M.R. & Brown, M.J. (Eds): TASMANIAAND THE SOUTHERN OCEAN. Pap. Proc. R. Soc. T asm. 133(3): 51-56. ISSN 0080- 470). Antarctic CRC, GPO Box 252-80, Hobart, Tasmania, Australia 7001 (NLB, SRR, RM); and CSIRO Division of Marine Research, GPO Box 1538, Hobart, Tasmania, Australia 7001 (SRR). Antarctic B~ttom Water is the coldest and densest water found in the global ocean. It spreads into all the major ocean basins, carrying the cold water towards the equatorial regions, and is a central component of the global thermo-haline circulation. However, the mechanisms of bottom water formation are not well established; its geographical distribution and rate of formation have yet to be fully quantified. Polynyas, which are large persistent openings in sea-ice that form during the winter near the Antarctic Coast, playa central role in the formation or Antarctic Bottom Water. This paper describes the bottom water formation around the Antarctic continental margin with particular emphasis on the processes and mechanisms of the Adelie Land Bottom Water formation near Dumont D'Urville south of Tasmania. Key Words: Antarctic Bottom Water, polynya, brine rejection, Adelie Land Bottom Water. INTRODUCTION Australian-Antarctic basin than in either the Weddell or ~ ~. Ross Seas. Antarctic Bottom Water (AABW) is one of the most Bottom waters are an important part of the thermo­ important water-masses in the global ocean. It is the coldest, haline circulation, and increased greenhouse gas scenarios densest water found in the deep ocean. This water is found from coupled-ocean-atmosphere models show that the rate in all of the deep basins around the Antarctic continent, of bottom water formation is expected to decrease (Manabe broken only by the relatively shallow sill through Drake et al. 1990, Manabe et al. 1991). It is important that these Passage. Estimates of the volume of AABW (defined by model scenarios are validated with observations. In this water denser than neutral density - Jackett & McDougall paper the geographical distribution of ADLBW in the 1997 [yn> 28.27 kg.m-3]) show it to occupy 3.5% of the Australian-Antarctic basin, evidence for its time variability volume of the ocean (Orsi et al. 1999) and affect, through of formation and its links to the Mertz Polynya are described. mixing and advection, more than 41 % of the global oceanic volume (Worthington 1981). Estimates of the production rate of bottom water vary but range from 8-12 Sv (Orsi et BOTTOM WATERS IN THE al. 1999 [1 Sv=106 m3.s-1]), giving a renewal time (defined AUSTRALIAN-ANTARCTIC BASIN here as the volume occupied by AABW divided by production rate) of 180-120 years, respectively. This renewal time is Describing the precise distribution of bottom waters and quite long, reflecting the fact that the production rate is estimating their production are essential requirements for small compared with the volume of the ocean occupied by understanding the mechanisms and sources ofbottom water AABW. Yet it is this production rate combined with the formation. The CFC-ll concentration at 10 m above the relatively low mixing rates which creates the strong density ocean floor (fig. 1) is taken from hydrographic data obtained gradients that define the Antarctic Circumpolar Current. on a voyage of the RSV Aurora Australis from January­ The main formation of AABW is thought to occur in March 1996 (see Bindoff et al. 1997 and Rosenberg et al. three distinct regions, on the continental shelves of the 1997 for a more complete description of this voyage). These western Weddell Sea, western Ross Sea and Adelie Land, CFC-ll measurements are accurate to ~ 1% (WOCE south of Tasmania. The Adelie Land source of bottom standard). Although the source ofCFC-ll in the atmosphere water has always been considered to be negligible (Carmack is increasing, the rate of increase from 1990 to 1996 is just 1977) compared with the Weddell and Ross Sea sources. 6% or about 1% per year (Gras et aL 1999). Thus, for the However, recent analyses of the volume of the AABW relatively young waters directly above the ocean floor and characteristic of these three source regions show that the the shelf waters presented here « 10 years old) there is no Weddell, Adelie and Ross sources are respectively 68%, need to adjust the CFC-ll values to allow for the time 24% and 8% by volume (Rintoul 1998), that is Adelie variation in the atmospheric source, and for these data this Land Bottom Water (ADLBW) is the second largest source. adjustment is small compared to the effects of mixing. The This new result is also supported by a census of Chloro­ CFC-ll Chloro-fluorocarbons are transferred from the fluorocarbon (CFC-ll) concentrations around the Antarctic atmosphere to the oceans through the surface of the ocean. continent (Orsi et al. 1999), which shows that layer Because CFCs are chemically passive in the ocean, they act of AABW defined by the CFC-ll is thicker in the as a tracer or dye. The highest concentrations (to first order) 52 NL. Bindoff ,S,R. Rintoul and R. Massom Bottom CFC11 (pm/kg) 62 63 64 66 67 6~0 90 100 110 120 130 140 150 160 Longitude °E FIG. 1 -~ The CFC11 concentration (pmollkg) taken from the deepest water sample (typically 10 m from bottom) for all eTD stations over the deep ocean. The numbers are the observed CFC-11 concentration at each CTD. The size ofthe circle gives the amplitude of'the CFC11 concentration. The 1000 and 3000 m depth contours are shown. These data come from the MARGINEX experiment (Bindoffet al. 1997), obtained in January-March 1996. The thick dashed contours show the ice shelves in this region. can be interpreted here as waters that have been most dense) water flowing downslope in either of these two recently in contact with the atmosphere. Each of the north­ sections. This suggests that the source waters are flowing south hydrographic sections (except at 1500E) cross the down the continental slope during the winter or between continental shelfbreak shown by the 500 m isobath. On the the two sections or in discrete canyons on the continental shelf floor, very high CFC-ll concentrations of greater than slope (Rintoul 1998). 4 pmol.kg-1 occur, consistent with the moderately rapid The seasonal variability of bottom water formation is overturning of the shelf waters and mixing with the poorly known because the extensive sea-ice cover during atmosphere. However, these concentrations decrease very winter makes the continental slope and shelf region largely rapidly down the continental slope. In the deep ocean, the inaccessible to conventional ship-based measurements. lowest deep-water CFC-ll concentrations occur in the west However, temperature measurements from 10 m above the (along 800E) with concentrations less than 1 pmol.kg-1, and ocean bottom from a mooring at 65°S, 1400E in 2600 m these concentrations progressively increase eastward until a of water show a distinctive seasonal signal. The warmest local maximum in CFC-ll concentration occurs at 1400E. temperatures occur during the February-June period and At 1500E the CFC-1 I concentration decreases again. the strongest cooling in the August-December period These lower CFC-l1 concentrations are also accompanied (Fukamachi, pers. comm.); this is consistent with the by higher salinities and warmer temperatures, consistent strongest formation being during the late winter-spring. with this water originating from the Ross Sea and flowing In addition, the temperature and salinity characteristics westwards along the continental rise (Gordon & Tchernia of shelf waters during the January-March 1996 voyage of 1972). The increase in CFC-11 at 1400E also accompanies the RV Aurora Australis show that the shelf salinities (fig. 2A) water that is colder, fresher and more oxygen-rich, implying are too fresh to form bottom waters, supporting the that the bottom waters at 1400E have been mixed with conclusions from the moored temperature measurements. more recently ventilated water originating from the Although the Ross Sea Bottom Water (RSBW) has continental shelf somewhere between 140° and 1500E. distinctive temperature and salinity characteristics present Excluding the large values of CFC-11 over the continental at 1500E, there is no evidence for this signature at 128°E shelf! slope break (less than 1000 m), all of the north-south (fig. 2A), where the bottom waters are colder and fresher sections show the highest values of CFC-11 (and also the (labelled as ADLBW). Here, the temperature-salinity coldest, freshest and highest in oxygen) offshore in waters correlation for waters < O°C forms a straight line that is deeper than 3000 m (fig. 1). Although it appears from noise free (fig. 2A). The shelf waters have a temperature these data that the source must be between 140° and near the surface freezing temperature (-1.85°C) and a 1500E, it is clear that during the summer time there is not salinity less than 34.5 psu. For this section there is no a continuous plume of high CFC (cold and fresh and simple two-end member mixing scheme between the Bottom water formation and polyn,yas in Antarctica 53 summer shdf waters with the Modified Circumpolar Deep and 0.5°C) and is present along the entire slope between Water (labelled MCDW) and the ADLBW. By contrast, 140° and 1500 E (fig.
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